Aspects of the disclosure relate generally to problems caused by separating and removing harmful material, such as but not limited to sand, in the fiber line of a chemical pulp mill during production of chemical pulp. Specifically, equipment and methods to remove sand from the recirculation liquor of the fiber line (digester) feed system are disclosed.
Comminuted cellulosic fiber material treated in a conventional pulp production process, e.g. wood chips, biomass, or other comminuted cellulosic material (for convenience referred to here as “chips”) typically contain non-cellulosic harmful material. The non-cellulosic harmful material includes sand, dirt, stones, different pieces of metal, (e.g. nails, pieces of metal wire, or bolts and nuts), chips of metal, or other heavy cellulosic materials (e.g. knots). The density of this non-cellulosic harmful material is typically at least 10% higher than the density of the cellulosic material being treated (e.g. at least 50% higher). Much of this non-cellulosic harmful material can be removed during the process of producing the chips for feeding to the pulping process, but part of the non-cellulosic harmful material will be passed into the digester feed system, to an optional impregnation vessel, and to the digester itself.
In many existing digester feed systems, smaller non-cellulosic harmful material such as sand, dirt, stones, etc., remain with the recirculation liquor used within the digester feed system. For simplicity purposes, the term “sand” is used throughout this document to mean smaller non-cellulosic harmful material previously identified. The recirculation liquor (for convenience referred to here as “liquor”) typically is used as transport liquor in digester feed systems having low pressure feeders, high pressure feeders, chip pumps, slurry pumps, or similar equipment where liquor may be needed.
Liquor within the digester feed system may have many functions and many sources. For clarity purposes, “entrained liquor” in this disclosure refers to liquor fed to the sand separator vessel and contains sand and even other solid particles.
The liquor is recirculated within the digester feed system to assist or improve the transport of chips in the digester feed system. Liquor may be any suitable sort of liquid, including but not limited to fresh water, fresh cooking liquor, black liquor, green liquor, recirculated liquor, cooking solution, or any combination thereof.
Usually sand is separated from the liquor in the digester feed system by means of some sort of separator. A conventional sand separator vessel removes sand and other solid particles from the entrained liquor circulated via a chip conveyor arrangement at the digester plant. U.S. Pat. No. 4,280,902 describes a cyclone-type sand separating device for removing undesirable solid particles, especially sand, from the debris-laden, or entrained liquor. Removal of solid particles, particularly sand, from the entrained liquor results in an essentially sand free (cleaned) liquor. The essentially sand free (cleaned) liquor may then be returned and become a part of the liquor of the digester feed system. For simplicity, the solid particles removed from the entrained liquor will be referred to as “sand,” although other undesirable solid particles may also be present.
A sand separator vessel typically is generally cylindrical in shape, typically with an upper section and a lower section, and comprises a tangential inlet into a cylindrical container. The cylindrical container is generally comprised of an upper section, generally having a cylindrical shape, and a lower section, generally having a cone or conical shape. In this document the upper section of the cylindrical container may also be referred to as a “cylindrical portion.” In such devices, the debris laden liquor is introduced into an upper section of the cylindrical container, and a forceful whirling motion is generated within the cylindrical portion. Sand, separated out of the entrained liquor and driven by gravitational and centrifugal forces, is forced downward in the cylindrical portion into a lower section, which may be a separating chamber, usually having a conical shape. From the lower section or separating chamber, the sand is discharged.
Within the lower section or separating chamber is located a funnel-shaped replaceable insert. For simplicity, in this application the term “replaceable insert” and “insert” may be used interchangeably. The replaceable insert has openings to allow sand to pass through the replaceable insert into the solid particle collection region (collection region) and ultimately out of the sand separator vessel.
The location of the replaceable insert within the sand separator vessel is at a level to prevent vertical stagnation of sand while having minimal disruption of the whirling motion. An outlet for the essentially sand-free (cleaned) liquor is located in the upper part of the cylindrical portion.
Typically, replaceable inserts are perforated metal pieces shaped like a funnel. The perforations may be vertical slots or circular holes and may extend the entire surface of the replaceable insert. Replaceable inserts are attached to the inner wall of the lower section of the sand separator vessel (within the separating chamber). As the entrained liquor enters the sand separator vessel, the whirling motion within the sand separator vessel separates the liquid and solid particles (sand) within the entrained liquor. Sand moves downward to the lower section, resulting in a strong whirling flow with complex flow patterns through the openings in the replaceable insert and eventually out a discharge of the sand separator vessel.
The replaceable insert is typically conical (or funnel) in shape with a large opening at one end and a small opening at the opposite end. Generally the replaceable insert is attached at the large opening end to the interior wall of the sand separator vessel in the vertically lower region of the sand separator vessel. A rigid attachment is typically used to connect the replaceable insert to the interior wall of the sand separator vessel.
Several adverse operational conditions have been identified with the sand separator vessel. As a result of the whirling motion and centrifugal and gravitational forces needed to separate the sand from entrained liquor, replaceable inserts become damaged and can collapse in on themselves. Additionally, uneven wear of the replaceable insert is experienced.
Uneven wear results in some sections of the replaceable insert being completely absent while other sections remain intact. Where the replaceable insert is worn through (or absent), sand may violently contact the collection region wall on the interior of the sand separator vessel and cause erosion which leads to material thinning, weak spots, and eventually holes, in the sand separator vessel.
Undesirable and complex flow patterns of the sand within the replaceable insert exist causing adverse operational conditions such as the clogging of openings in the replaceable insert. The undesirable flow patterns cause sections or regions of very high sand particle velocities within the replaceable insert where the openings in the replaceable insert become overwhelmed and clog.
These adverse operational and uneven wear conditions at some point in time require the sand separator vessel to be taken offline and the replaceable insert repaired or replaced. There is a long felt need to provide a replaceable insert more readily capable of withstanding gravitational and centrifugal forces, undesirable sand flow patterns and velocities, as well as the abrasive attack of the sand moving within the replaceable insert. It is to these needs and others that the present disclosure is directed.